Dear Steven,
thank you for your reply!
The printed results of my excectuted code are:
opencv_error: 0.31797973380564853
opencv_numpy_error: 0.31797973380564853
mean_error_alternative: 0.5015439217802148
It is computing the Euclidian distance, but it is handling it as if it is computing the error for a 6D problem (sqrt((1.1-1)^2 + (1.2-1)^2 + (0.3-0)^2 + (1.4-1)^2 + (1.5-1)^2 + (0.6-0)^2)/3) and is making no difference between x and y values.
However, in the example I posted we have only three corners and the re-projection error is defined as the 2D difference between a found point (e.g. x,y = (1.1,1.2)) and projected point (e.g. x,y = (1,1)) which would be sqrt((1.1-1)^2 + (1.2-1)^2) for the first point, which imo has to be repeated for all three point pairs and then averaged.